1. Field of the Invention
The invention is directed to a printing unit of a printing press and, more particularly, to a web press constructed as a periodical printing press.
2. Description of the Related Art
A printing unit of a web press constructed as a periodical printing press typically has two printing couples, where each printing couple comprises a transfer cylinder, a form cylinder, an inking unit and a dampening unit. The transfer cylinders are also called blanket cylinders, and the form cylinders are also known as plate cylinders. The transfer cylinders of the printing couples in a printing unit of this kind roll upon one another while forming a printing nip through which printing stock to be imprinted in the printing unit is conveyed in horizontal direction.
It known to provide each printing couple in a conventional printing unit with a separate drive motor that drives either the form cylinder or the transfer cylinder of the respective printing couple. Here, the drive motors of a drive unit preferably drive the transfer cylinders of the respective printing couples, and the form cylinder is then driven by way of the transfer cylinder.
In such a conventional printing unit, the mass inertia of the drive motor, together with the mass inertia of the cylinders to be driven by the drive motor, form an oscillating system having a certain natural frequency. When this oscillating system is excited by a frequency on the order of the natural frequency, vibrations develop which negatively impact the printing quality. Therefore, the drive controller of the respective drive motor is designed in such a way that it does not excite the oscillating system with its natural frequency. This is achieved in that the controller filters out, and therefore does not correct, interference in the load torque curve in the range of the natural frequency of the oscillating system.
However, since fluctuations in torque, which are not detected because of the filtering in the drive controller of the drive motor and can therefore not be corrected, can also act on the drive motor in the neighborhood of the natural frequency of the oscillating system, doubling can develop in the print area, which impairs printing quality.